MAC, A Verified Static Information-Flow Control Library

The programming language Haskell plays a unique, privileged role in information-flow control (IFC) research: it is able to enforce information security via libraries. Many state-of-the-art IFC libraries (e.g., LIO and HLIO) support a variety of advanced features like mutable data structures, exceptions, and concurrency, whose subtle interaction makes verification of security guarantees challenging. In this work, we focus on MAC, a statically-enforced IFC library for Haskell. In MAC, like other IFC libraries, computations have a well-established algebraic structure for computations (i.e., monads) responsible to manipulate labeled values—values coming from an abstract data type which associates a sensitivity label to a piece of information. In this work, we enrich labeled values with a functor structure and provide an applicative functor operator which encourages a more functional programming style and simplifies code. Furthermore, we present a full-fledged, mechanically-verified model of MAC. Specifically, we show progress-insensitive noninterference for our sequential calculus and pinpoint sufficient requirements on the scheduler to prove progress-sensitive noninterference for our concurrent calculus. For that, we study the security guarantees of MAC using term erasure, a proof technique that ensures that the same public output should be produced if secrets are erased before or after program execution. As another contribution, we extend term erasure with two-steps erasure, a flexible novel technique that greatly simplifies the noninterference proof and helps to prove many advanced features of MAC.